1. Field of the Invention
The present invention relates to an apparatus for correcting a time base error of a video signal.
2. Description of the Prior Art
It is well known that a video signal reproduced by a video tape recorder (VTR) is more or less fluctuated on the time base (time base error) caused by a reproduction mechanism of the VTR.
There is a conventional time base error correction apparatus for compensating the fluctuation (jitter), which has so-called ring type memories (which will be described later). In the conventional apparatus, reproduced video signals are written in memories with a write clock signal synchronizing with the reproduced video signals and then the written reproduced video signals are read from the memories with a read clock signal of a constant frequency.
However, there are disadvantages in the conventional apparatus as follows. Firstly, in the case where a frequency of the write clock signal is higher than that of the read clock signal, a write timing may catch up with and get ahead a read timing. Secondly, in the case where a frequency of the read clock signal is higher than that of the write clock signal, the read timing may catch up with and get ahead the write timing.
The disadvantages will be explained in detail with FIGS. 1(a) to 1(c). In each drawing, an inner part M of a circle depicts a memory area in or from which reproduced video signals are written or read.
FIG. 1(a) shows a usual relationship between a write timing W and a read timing R. As shown, both timings are separated far from each other on the time base. The write timing W may be fluctuated within a period WW, because the write clock signal synchronizes with the reproduced video signals. This write timing is, however, separated far away from the read timing, thus inducing no problem.
FIG. 1(b) shows the case where the frequency of the write clock signal is higher than that of the read clock signal, so that the write timing W catches up with and gets ahead the read timing R. As shown, a write timing W1 gradually goes near a read timing R1 and finally a write timing W2 catches up with and gets ahead a read timing R2. Accordingly, before a first reproduced video signal is read from a part of the memory area M, the next second reproduced video signal will be written in the same part. This results in that the first reproduced video signal cannot be read from the memory area, so that the number of horizontal scanning lines corresponding to the first reproduced video signal per field will decrease.
Contrary to this, FIG. 1(c) shows the case where the frequency of the read clock signal is higher than that of the write clock signal, so that the read timing R catches up with and gets ahead the write timing W. As shown, a read timing R3 gradually goes near a write timing W3 and finally a read timing R4 catches up with and gets ahead a write timing W4. Accordingly, a reproduced video signal which has already been read from a part of the memory area M is again read from the same part. This results in that the number of horizontal scanning lines corresponding to that twice reproduced video signal per field will increase.
Accordingly, when the number of horizontal scanning lines per field increases or decreases after a vertical synchronizing signal, as described above, vertical synchronization on a screen will be disturbed in a period of time before the next coming vertical synchronizing signal, thus the picture quality on the screen will be remarkably deteriorated.